A conventional stepping motor 1, for example, as shown in FIG. 9 which is used for driving a lens in a digital camera, a digital video camera and the like has been known.
The motor 1 shown in FIG. 9 includes a stator 2 and a rotor 4 which is disposed on an inner side of the stator 2 and provided with a rotor shaft 3.
The stator 2 holds a bobbin 7 with an outer yoke 5 and an inner yoke 6. A winding wire 8 is wound around the bobbin 7.
A rotor magnet 9 is fixed to the rotor shaft 3 and one end of the rotor shaft 3 (the other end is not shown) is supported by a bearing 11 through a ball 10.
The bearing 11 is made of resin and a recessed part 11b for holding the ball 10 is formed in a rotor shaft support face 11a located on the rotor shaft 3 side. The bearing 11 is fitted into a hole 12a of a cap part 12 made of resin which is fixed to the stator 2. In addition, the bearing 11 is urged toward the rotor shaft 3 by a urging member 13 which is disposed on the one end side of the cap part 12 (see, for example, Japanese Patent Laid-Open No. 2002-191150).
As shown in FIG. 10, a plurality of leg parts 13a (four parts in this embodiment) which is formed by bending a metal plate is formed in the urging member 13. The leg part 13a is, as shown in FIG. 9, engaged with the cap part 12 and held by the cap part 12. Further, a center portion of the urging member 13 is stamped out in a substantially U-shape and an inner side portion is raised or bent and thus an urging piece 13b comprised of an elastic tongue piece is formed in the urging member 13. In addition, a tip end (free end) of the urging piece 13b is bent so as to be elastically abutted with a pressurization face 11c of the bearing 11 to urge the bearing 11 toward the rotor shaft 3 side.
A center portion of the bearing 11 used in such a motor is urged by the urging member 13 and thus it is considered to be preferable that an injection gate trace for injecting resin material when the bearing 11 is resin-molded is not located at the center portion of the bearing 11.
Therefore, when a cylindrical object such as the above-mentioned bearing 11 made of resin is molded, for example, as shown in FIG. 11, it is commonly performed such that resin material is injected from three equally divided positions 14 which are disposed closer to an outer periphery of the bearing 11, or that resin material is injected from an outer periphery of the bearing 11 by using a ring-shaped injection gate.
However, in the case that a cylindrical object made of resin is used as the above-mentioned bearing 11, when its circularity is not precise, the bearing 11 cannot be fitted into the hole 12a of the cap part 12 or a gap space is formed between them. Especially, in the bearing 11 used in a motor in which miniaturization is required, molding precision (circularity) for the bearing 11 greatly affects on rotational accuracy of the rotor shaft.
Moreover, in the case when a ring-shaped injection gate is used, a die structure becomes to be complicated and an extra working for removing an injection gate trace is required and thus its manufacturing cost increases.
Further, the urging piece 13b of the urging member 13 is formed such that a center portion of the urging member 13 comprised of a metal plate is stamped out in a substantially U-shape and its inner side portion is raised and bent.
Therefore, in order to secure an urging width of the urging piece 13b to the bearing 11, a width of the urging piece 13b is required to be broadened. However, when the width of the urging piece 13b is broadened, its rigidity (elastic force) is increased and thus the bearing 11 is urged toward the rotor shaft 3 side with a larger urging force than that is required.